A review on the efficacy of Ocimum gratissimum, Mentha spicata, and Moringa oleifera leaf extracts in repelling mosquito

@@@In recent times, repellents and synthetic drugs have been identified as having negative toxicity effects on humans and the environment. Apart from the unfavourable effects on man and livestock caused by these chemicals-based (synthetic) repellents, they are also expensive, non-biodegradable, and no more effective because mosquitoes are getting adapted. With these drawbacks, an eco-friendly plant-based insecticide as a substitute is needed urgently. This paper reviews the extraction and use of essential oil from the leaves of Mentha spicata, Ocimum gratissimum, and Moringa oleifera as mosquito repellent. Carvone, Eugenol, and 9-Octadecenoic acid were discovered to be the most active components in the M. spicata, O. gratissimum, and M. oleifera extracts, respectively, using gas chromatography-mass spectrometry (GC-MS). In recent times, repellents and synthetic drugs have been identified as having negative toxicity effects on humans and the environment. Apart from the unfavorable effects on man and livestock caused by these chemical-based (synthetic) repellents, they are also expensive, non-biodegradable, and no more effective because mosquitoes are getting adapted. An eco-friendly plant-based insecticide as a substitute is needed urgently. Diseases transmitted by mosquitoes are still a significant reason for the global mortality rate, with over 700 million individuals experiencing such diseases every year. With the proper formulation of other repellent forms using their oils, they can replace non-degradable synthetic mosquito repellents since they are eco-friendly. In general, the mosquitocidal activity and percentage protection of plant extract increase with increasing concentration of the extracts in different formulations. This paper is our original work. We certify that this manuscript has not been published in part or whole elsewhere in any language, and it has not been submitted to any other journal for reviews. In recent times, repellents and synthetic drugs have been identified as having negative toxicity effects on humans and the environment. Apart from the unfavorable effects on man and livestock caused by these chemical-based (synthetic) repellents, they are also expensive, non-biodegradable, and no more effective because mosquitoes are getting adapted. An eco-friendly plant-based insecticide as a substitute is needed urgently. Diseases transmitted by mosquitoes are still a significant reason for the global mortality rate, with over 700 million individuals experiencing such diseases every year. With the proper formulation of other repellent forms using their oils, they can replace non-degradable synthetic mosquito repellents since they are eco-friendly. In general, the mosquitocidal activity and percentage protection of plant extract increase with increasing concentration of the extracts in different formulations. This paper is our original work. We certify that this manuscript has not been published in part or whole elsewhere in any language, and it has not been submitted to any other journal for reviews.


Background
Mosquitoes cause inconvenience by their bites, as well as the transmission of deadly diseases [1]. Fewer than 10% of the roughly identified mosquito species are viewed as proficient vectors and pathogenic carriers of infections with significant effects on human health and well-being, both directly and indirectly.
Diseases transmitted by mosquitoes are still a significant reason for the global mortality rate, with over 700 million individuals experiencing such diseases every year [2].
Diseases carried by mosquitoes affect the economy, incorporating loss in business and work yields, especially in nations with marine and temperate atmospheres; Page 2 of 12 Ojewumi et al. Beni-Suef Univ J Basic Appl Sci (2021) 10:87 however, anywhere in the world is prone to vector-borne infections [3]. Various diseases like; Filariasis, Malaria, yellow fever, Japanese encephalitis, zika virus, Dengue fever, etc., are transmitted by mosquitoes [4]. Malaria is among the biggest public health issues globally. Especially in parts of Africa, in which Nigeria has the largest amount of cases of malaria [5]. However, over time, the constant application of synthetic insecticides has culminated in mosquito resistance. This is due to their adaptation and ability to develop resistance to the active ingredient of the chemicals and also changes in its metabolism and behaviour. Mosquitoes have compound forms of identifying their hosts, and various types of mosquitoes react to various stimuli. Some mosquitoes are active in the dawn and dusk, but during the day, there are also mosquitoes searching for hosts [6]. In other to prevent being bitten by mosquitoes, materials, or actions that attract them are advised to be avoided even as their repellents are being applied. And also, the acts that reduce the repellent's effectiveness should be averted [6].
Mosquito repellents generally function by hindering the capacity of the female mosquito to recognize the external stimuli (for example, Carbon-dioxide, water vapor, and heat) that she utilizes to spot a host [7]. Various synthetic and plant-based chemicals are available and are known to ward off mosquitoes. N,N-Diethyl-meta-toluamide, also called DEET, is the most frequently used synthetic chemical repellent. Since it became commercially available, it has been implemented above one million times in the 40+ years of its existence. DEET-based insect repellents cause irreparable harm to the environment since they consist of chemicals that are not easily degraded, and their associated neurotoxicity [8]. An alternative to repelling mosquitoes could be plant-based natural materials like plant oils to prevent the adverse effects of synthetic repellents. In comparison with synthetic repellents, they are deemed safe and good for the environment [9].
Before the application of synthetic substances, the repellent features of plants on mosquitoes as well as other pest insects were well known. Humans historically used natural compounds to shield themselves from insect bites [10]. Since ancient times, man has utilized plant parts and secondary plant metabolites for pest control. During the pre-DEET period, vector mosquito reduction relied primarily on the environmental control of the breeding surroundings, i.e., depleting the source. During this time, Pyrethrum, Anabasine, Quassia, Camphor, Turpentine, Nicotine, Derris, D-limonene, Hellebore, Azadirachtin, and Chrysanthemum were some plant-based (botanical) insecticides used in different countries [11].
As a possible source of insecticidal materials, the plant kingdom has been of considerable interest. Many plant kingdom species produce a variety of secondary metabolites that perform a crucial role in plant defence against mosquitoes/insects. Plants are a great source of biologically active chemicals/compounds and can be a substitute source for mosquito repellent products [12]. Furthermore, unlike regular insecticides that consist of a sole active ingredient, insecticides derived from plants contain botanical mixtures of chemicals/compounds that work collectively on processes, both physiological and behavioural [13].
Biologically active compounds derived from selected plants species such as Ocimum gratissimum (O. gratissimum), Hyptis sauveolen (H. sauveolen), Acarcia arabica, Azadirachta indica, and Eleusive indica have been commonly used in the past to control insects in many tropical counties [14]. The use of compounds from plants for controlling mosquitoes has been accounted for since 1933, and these chemicals in their essential oils obtained from the various plants have shown to be a feasible alternative to synthetic repellents and also exhibit good repellence against mosquitoes [15]. Extracts and essential oil from plants can be a replacement for synthetic insecticides since they are efficient, easily decomposable, environmentally friendly, and relatively cheap.
Essential oils, which are volatile substances, have an oily scent and are derived from the various parts of the plant [16]. They are extracted with diverse methods, and plant parts used for their isolation include leaves, stem bark, flowers, and roots [17]. 1. Aerosol-The most commonly used form of repellents for mosquito is aerosol. It comprises of a propellant, a solvent, and other components. The active ingredient is diluted to a particular concentration by ethanol or propyl alcohol, the solvent. It also holds the mixture of all the materials, so that even after extended storage, the product will still be active. They are packed and discharged as a spray under pressure [18]. 2. Cream-Repellent creams for mosquito make the individual unattractive to the biting mosquitoes. The smell of the skin and human breathe attracts mosquitoes and other insects that feast on blood. Carbon dioxide is released by humans once they breathe out, and this draws insects. The skin is rendered unattractive to mosquitoes by the application of this repellent cream Thus, the mosquitoes will be seen flying around, but it will not bite you [19]. 3. Stick-These repellent incense sticks are plant-based and DEET-free. The sticks use a combination of citronella, rosemary, peppermint, lemon grass, cedar wood, and bamboo to ward off mosquitos [20]. They release fragrant smoke when they are burnt, which also wards off mosquitoes around the place of burning. 4. Mosquito coil-A mosquito coil is an incense for repelling mosquito, usually shaped into a spiral. It is generally kept in the middle of the spiral, holding it in the air. Its working mechanism is also identical with the incense stick, and candle as its burning normally starts at the far end of the spiral and moves toward the middle of the spiral, creating smoke that repels mosquitoes [21].
There are many mosquito repelling plants, both wild and cultivated. It is imperative to note that the repelling

Mentha spicata
Mentha spicata, also known as spearmint, is a genus from the Lamiaceae (mint) family, and plants belonging to this family are a great source of polyphenols with good antioxidant features [22]. Spearmint essential oils have often been utilized in different ways, like plant diseases and insect pest control, in conventional medicine, and also in cosmetics and culinary [23]. Spearmint is native to northern England and is cultured in tropical to moderate climate zones like Europe, South Africa, America, China, and Brazil. These days, it is grown extensively all around the world [22]. Because of its invasive, creeping rhizomes, gardeners usually plant it in containers or planters, and its leaves can still be used clean, frozen, or dry [24]. The most copious chemical in spearmint oil is R-carvone, giving its distinctive scent to spearmint [25,26]. Spearmint essential oil was successful as a mosquito larvicide. Because of its toxicity and negative environmental effect, the application of spearmint as a larvicide would be a better substitute for conventional insecticides [25]. Carvone is noted to have the potential of preventing the growth of bacteria, and also acting as an insect repellent and fungicide [22]. Spearmint leaves were chosen because its essential oil is a good natural insect repellent from past work, and its distinctive smell will improve the quality of the repellent forms (Fig. 1).

Moringa oleifera
One of the most commonly grown species of a monogeneric family-the Moringaceae is Moringa oleifera (M. oleifera), and it's indigenous to India's sub-Himalayan regions [27,28]. Moringa is cultivated predominantly in semi-arid, tropical, and temperate regions. In dry sandy soil, it thrives best, tolerating poor soil, and also in coastal regions. It develops in nearly all types of welldrained soils and is resistant to drought and retains water throughout the dry season by shedding of leaves. It is usually grown and naturalized in Mexico, Central and South America, Sri Lanka, tropical Africa, Philippines, India, and Malaysia. [29].
The Moringa tree is deemed among the world's most valuable trees; nearly every portion can be used for    [31,32]. The phytochemicals present in M. Oleifera can function as larvicides, repellents, arthropod growth controllers, and also possess a very deterrent behaviour as noticed by many researchers and analysts. Seed extract from M. Oleifera has a good effect on malaria and has no adverse impact on humans [33]. Moringa plant extracts possess repellent properties and have an extensive range of medicinal applications. Different portions of this plant, like the leaves, fruit, seed, flowers, bark, and roots, contain a list of important purposes [33] (Fig. 2).

Ocimum gratissimum
Ocimum gratissimum, also called basil clove, African basil, is an Ocimum species from Africa, South Asia, and South America [35]. It is a species of tropical plants popularly referred to as "scent leaf, " as Nigerians love to call it. In native Nigeria, it is called Nchanwu leaf in (Igbo), Daidoya in (Hausa), and Efirin in (Yoruba). It is a tropical plant species belonging to the Labiatae family [36]. The leaf consists of a variety of nutrients and minerals that provide the plant with many health benefits [37]. It is stated that fresh basil leaves contain protein, magnesium, Anatol, Boron compounds, Eugenol, Tryptophan, Stigmasterol, zinc, Tannin, and Cinnamic acid [38].
They are often used to make soups as a local spice and flavor. O. gratissimum's essential oil primarily contains eugenol, which provides some signs of antibacterial activity as well as a strong mosquito repellent due to its smell  and insect toxicity [39,40]. Thus, Scent leaf can be mixed into mosquito coils, incense, creams, or ointments to ward off reptiles and insects [41]. It is an herbaceous plant that grows with an upright stem reaching six feet high [42]. Scent leaf, which is a potential repellent plant, was considered for the study as it is cheap, easily accessible (as can be found in the backyard of most homes or local markets) (Fig. 3).

Essential oil and extraction
The essential oil is a potent water repellent liquid that houses labile plant-based chemical compounds (easily evaporated at normal temperatures) [44,45]. Often known as volatile oils, they are also named according to the plant they were derived from, like clove oil, which is an essential oil obtained from the clove [46]. The metabolites such as the monoterpenes like camphor, eugenol, citronellol, terpinolene, α-pinene, citronellal, thymol, limonene, and cineole are the usual components in several essential oils showing mosquito repellent behaviour [46].
Extraction "means transferring compounds from a liquid or solid to another solvent or phase [47]. Two immiscible phases are combined to separate a solvent from one phase to the next, depending on the relative solubility in each phase. Extraction is a primary method used in plant materials for separating their compounds. Since essential oils are the liquefied form of the plant, instead of being produced in laboratories synthetically, they are obtained from materials of a plant by extraction methods appropriate for the specific plant component containing the oils. The common extraction methods for essential oil include; Soxhlet extraction, steam distillation, CO 2 extraction, water distillation, maceration, cold press extraction, and effleurage [48,49].

Characterization of the essential oils
Characterization involves the description of the distinctive nature or features of the extracted oils. Characterization is based on different analyses for the extract, such as phytochemicals, gas chromatography-mass spectrometry [GC-MS], and Fourier Transform Infrared Spectroscopy [FTIR] analysis.

GC-MS analysis
One of the presumed hyphenated analytical techniques is gas chromatography-mass spectroscopy (GC-MS). GC-MS is an analytical tool incorporating the features of gas chromatography and mass spectrometry to determine various components in a test sample [50]. Gas chromatography isolates the components of a mixture, and each component is individually identified by mass spectroscopy.
GC-MS is used to analyse organic compound mixtures that are unknown, and its application in determining the composition of bio-oils extracted from raw biomass is a key use of this technology [51].

FTIR analysis
Fourier-transform infrared spectroscopy (FTIR) is used to acquire from a solid, liquid, or gas sample its infrared absorption or emission spectrum. At the same time, an FTIR spectrometer obtains data with high spectral resolution over a broad spectral range [52]. According to [53], "FTIR screening is essentially an experimental analysis technique used to distinguish organic and some inorganic substances by applying infrared radiation (IR). " The FTIR instrument delivers about 10,000 to 100 cm −1 infrared radiation over a sample, absorbing some radiation and passing through some. Vibrational energy is produced by the conversion of absorbed radiation [54]. Table 1 shows the comparative analysis of the three (3) Table 7 which shows Carvone having the highest composition and most prevalent in M. spicata oil corroborates with the result of [66] in which carvone was discovered as the most sufficient compound/component in the extract. The extracts also showed the presence of various biologically active phytocomponents in the GC-MS analysis. The presence of these photo components also contributes to the observed medicinal property in addition to the antimicrobial activity of the plant. Among the three leaves, O. gratissimum extract is the best repellent plant, followed by M. spicata and M. oleifera. This is due to the high composition of eugenol (which is a phenol) in the extract as it has been reported not only to control insects like mosquitoes but also to provide a knockout effect on them [67]. The active component of clove oil, eugenol, is a fast-acting contact insecticide that is effective on a wide variety of insects/pests such as mosquitoes, ants, cockroaches, etc. [68]. Eugenol has little or no residual activity other than a lingering scent of cloves as mosquitoes detest the smell of cloves [69]. 9-Octadecenoic acid, which is the most prevalent compound in Moringa leaf oil extract, exhibited moderate repellent activity at 30 min after treatment, according to [70]. According to [12], leaf extracts of M. oleifera leaves possessed poor repellent activity, and this corroborates with the result of [70]. Table 7 Chemical composition of Mentha spicata L. essential oil. Source: [65] The bold signifies the major component identified by GC-MS, others are the identified sub-components at different retention times which summed up to the total discovered in bold a,b Means of identification of samples

Conclusions
The initial findings of the laboratory assessment from the previous works show the repellent potential of M. spicata, O. gratissimum, and M. oleifera leaves against mosquitoes. With the proper formulation of other repellent forms using their oils, they can replace non-degradable synthetic mosquito repellents since they are eco-friendly. In general, the mosquitocidal activity and percentage protection of plant extract increase with the increasing concentration of the extracts in different formulations.
The Soxhlet extraction technique is a conventional and most preferable method for obtaining plant extracts easily. However, the result from the efficacy tests revealed that natural repellents derived from plant extracts tend to provide protection for a shorter time. The active component of the M. spicata extract responsible for its repellent activity is carvone. While that of O. gratissimum and M. oleifera are Eugenol and 9-Octadecenoic acid, respectively. O. gratissimum essential oil is the best repellent plant, and its incorporation into a mosquito coil will offer the best protection against mosquitoes in comparison with the other plants and repellent forms.
Finally, the study establishes and reaffirms the potential of applying indigenous Nigerian plants' oil extracts with insecticidal properties for Mosquito control.
Authors' contributions MOE conceived the experiment and did the write-up, ORO proof-read the manuscript and DFA collated data. All authors read and approved the final manuscript.

Funding
This research received no external funding.

Availability of data and materials
Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.